1. Field of the Invention
The present invention relates to a valve timing changing apparatus for an internal combustion engine, which changes valve timings of intake and exhaust valves of the engine in accordance with, for example, an operational state of the engine.
2. Description of the Related Art
Intake and exhaust valves of internal combustion engines are reciprocally driven in accordance with the rotation of cam shafts, so that intake and exhaust ports opening into a combustion chamber of the engine are synchronously opened or closed, respectively. In a generally employed internal combustion engine, profiles of cams attached to the cam shafts determine the timings at which those valves are opened or closed, namely, valve timings.
On the other hand, some recently developed internal combustion engines are equipped with a valve timing changing apparatus for changing valve timings in accordance with an operational state of the engine. For example, this valve timing changing apparatus is designed to change valve timings such that, when the internal combustion engine is in an idle driving state, the valve overlap period during which intake and exhaust valves are simultaneously opened is decreased, and, when the engine is in a high-load driving state, the valve overlap period is increased. By changing valve timings in accordance with an operational state of the engine in this manner, it is possible to achieve a stable idle driving state and to enhance engine output as well as the intake efficiency during high-load driving.
For example, as such a valve timing changing apparatus, Japanese Patent Application Laid-Open No. HEI 5-118232 discloses "a valve timing control apparatus for an internal combustion engine". This apparatus employs variable valve timing mechanisms for both intake and exhaust cam shafts. These variable valve timing mechanisms are designed to change rotational phases of the intake and exhaust cam shafts so as to change the valve timings of intake and exhaust valves, respectively.
Because there is a certain limit to the control accuracy of the variable valve timing mechanisms of the aforementioned type, the actual valve timings constantly deviates from target valve timings. In other words, those variable valve timing mechanisms always suffer from control errors, which may be temporarily increased due to a possible response delay. Owing to such control errors, the valve overlap period substantially deviates from a desired length.
In order to change a valve overlap period to a length suited for an operational state of the engine, the aforementioned valve timing control apparatus wherein the valve timings of both the intake and exhaust valves are changed requires that both the variable valve timing mechanisms for the intake and exhaust cam shafts be controlled in such a manner as to set valve timings of the intake and exhaust valves to desired timings. However, in this case, respective control errors during valve timing control of the intake and exhaust valves are superposed on one another, so that the actual valve overlap period may further deviate from a target length.